Method for controlling beam formation in a mobile radio communication system and a base station therefor

ABSTRACT

A method is provided for controlling the beam formation of downlink signals, which are emitted in a mobile radio communications system by base stations of a first and a second radio cell to a mobile station, whereby the base stations are configured to emit a weighted downlink signal to the mobile station, using a weighting vector. The method includes the steps of determination of a weighting vector record in the mobile station and the base stations of the first and second radio cell; selection of an optimal weighting vector in the mobile station and transmission of a code word that is assigned to the optimal weighting vector to the base stations; and weighted emission of the downlink signal to each base station using the weighted vector that has been assigned to the code word. In the base stations, the weighting vectors are assigned to the code words in various ways, so that upon receipt of an identical code word, the base stations weight the downlink signal using different weighting vectors.

BACKGROUND OF THE INVENTION

The present invention relates to a method for controlling the beamformation of a downlink signal which is radiated by base stations of afirst and a second radio cell to a mobile station in a mobile radiocommunication system. The present invention also relates to a basestation suitable for implementing such a method.

In mobile radio communication systems such as the UMTS system, forexample, base stations having a number of antennas are employed toutilize space diversity. These antennas are weighted using differentweighting factors for data transmission to a mobile station; i.e., adownlink signal to be transmitted to the mobile station is applied tothe latter in each case multiplied by the weighting factor assigned tothe relevant antenna. The weighting factors are generally complexnumbers which include an absolute and a phase component. A radiationlobe in the direction of the location of the relevant mobile station isthereby produced at the base station for each mobile radio station in acell (beam formation). The weighting factors of the individual antennasare combined to form a weighting vector.

In so-called closed-loop transmit diversity schemes, the requiredweighting vector is estimated at the mobile station, quantized andtransmitted via the uplink dedicated physical control channel to thebase station where it is used for beam formation.

If the mobile station moves from a first radio cell of the mobile radiocommunication system to a second, a communication connection supportedby it must be switched from the base station of the first radio cell tothat of the second. This process is known as handover.

In the case of so-called soft handover, there is an intermediate statein which identical user data is transmitted in the downlink to themobile station from the base stations of two or more radio cells. Inindustry literature, a distinction is drawn between a soft handover inthe narrower sense whereby the radio cells each correspond to differentbase stations, and a softer handover whereby the radio cells correspondto different sectors of a base station. Where soft handover is referredto in the following, both alternatives will always be consideredincluded.

Normally, in the case of soft handover of a mobile station between twobase stations employing space diversity, a common weighting vector isselected for both base stations. This common weighting vector isdetermined in such a way that it maximizes the incoming power, at themobile station, of all the radio cells involved in the soft handover;i.e., a weighting vector W is sought for which the expressionP=W ^(H)(H ₁ ^(H) H ₁ +H ₂ ^(H) H ₂+ . . . )Wis maximized, where H1HHi is the covariance matrix of the transmissionchannel of the ith base station involved in handover, to the mobilestation. Consequently, all the base stations involved in the handoveruse the same weighting vector; i.e., they have the same spatialradiation characteristic.

FIG. 1 illustrates the above situation taking the example of theradiation lobes of two base stations BS1, BS2 and a mobile station MSlocated in the boundary region between the radio cells of the two basestations. As the two base stations BS1, BS2 apply the same weightingvector, their radiation lobes are identically oriented, and the mobilestation MS is, in each case, located at the edge of the two lobes andtherefore does not have optimum reception of the two base stations.

In order to increase the number of mobile stations that can besimultaneously supplied in a radio cell while at the same timeminimizing interference in adjacent cells, it is intrinsically desirableto increase the number of antennas at the base station in order to beable to produce more directive radiation lobes. As FIG. 2 shows, thisentails the risk of making a conventional soft handover impossible, asthe radiation lobes of both antennas no longer overlap in the boundaryregion of the cells. The mobile station MS here receives none of thedownlink signals of the base stations BS1, BS2 with sufficient quality.

An object of the present invention is therefore, to provide a method forcontrolling the beam formation of a downlink signal which enablesdownlink signals to be supplied to a mobile station which simultaneouslycommunicates with base stations of at least two radio cells, the mobilestation being simultaneously supplied with optimum quality from at leasttwo of the base stations, and also to specify a base station suitablefor implementing a method of this kind.

SUMMARY OF THE INVENTION

Accordingly, the method of the present invention involves the usualsteps of specifying a set of weighting vectors at the mobile station andthe base stations of the first and second radio cell, each weightingvector being assigned a code word, of selecting an optimum weightingvector from the vectors of the set at the mobile station andtransmitting the code word assigned to the optimum weighting vector tothe base stations, and of weighted radiating of the downlink signal toeach base station using the weighting vector assigned to the code word.

The distinctive feature of the present method is that the weightingvectors are differently assigned to the code words at the base stationsof the first and second radio cell so that these base stations weightthe downlink signal with different weighting vectors on receipt of anidentical code word. More precisely, if a mobile station transmits acode word which encodes a weighting vector which, if it is used by thefirst base station, provides the mobile station with optimum receptionof the downlink signal of the first base station, this weighting vector,if it is used by the second base station located at another site, iscertain to be non-optimum, wherein every other weighting vector of thespecified set is likely to be better.

Preferably, the set of weighting vectors is specified in such a waythat, in each case, it contains at least one weighting vectorcorresponding to a transmission path of the first radio cell and atleast one vector corresponding to a transmission path of the secondradio cell. Assigning an identical code word to these two vectors at therespective base stations of the first and second radio cells ensuresthat, by transmitting a single code word at both base stations, suitableweighting vectors can be selected specifically for the base stations ineach case.

If, in the foregoing and in the following, the discussion mainly relatesto two radio cells or base stations, this should not be taken to meanthat no more than two base stations can simultaneously radiate thedownlink signal. If the downlink signal is also radiated by a basestation of at least a third radio cell, the set of weighting vectorsmust include at least one weighting vector corresponding to atransmission path of at least a third radio cell and this weightingvector be assigned the same code word at the base station of theassigned third radio cell. Thus, the method according to the presentinvention can in principle, be extended to any number of radio cells orbase stations.

To define the set of weighting vectors, weighting vectors preferably aremeasured at the mobile station and transmitted to the base station in asequence whereby a first group of positions in the sequence is reservedfor the transmission of weighting vectors corresponding to atransmission path of the first radio cell, and a second group ofpositions in the sequence is reserved for the transmission of weightingvectors corresponding to a transmission path of the second radio cell.As such, on receipt of the set of vectors from the subscriber station,each base station is able, on the basis of the position that a vectoroccupies in the sequence, to detect whether this is a vector measuredfor the downlink signal of this base station and specifically determinedfor use at this base station, or a vector assigned to another station.

There also can, of course, also be third and further groups of positionsin the sequence if the subscriber station is intended to communicatesimultaneously with more than two base stations.

If the number of groups is variable, particularly if the number of basestations involved in a soft handover is variable depending on thereception conditions, it is advisable for the subscriber station totransmit information concerning the number of groups to the basestations, so that the base stations can infer, on the basis of thisinformation, which positions of the sequence belong to which group.

The positions of the various groups preferably follow one anothercyclically in the sequence; i.e., in the case of two groups, one groupincludes even-numbered and the other odd-numbered positions, in the caseof three groups, one includes the positions 1, 4, . . . , another thepositions 2, 5, . . . etc.

An alternative possibility is that the weighting vectors measured at themobile station are each transmitted to the base stations in conjunctionwith an indicator which designates the radio cell of the transmissionpath to which the weighting vector corresponds. As such, a weightingvector measured on any transmission path can be transmitted at eachposition of the sequence. In particular, each group may contain anynumber of vectors.

The two base stations preferably use two different rules for assigning aweighting vector transmitted in this way to a code word subsequentlytransmitted by the mobile station.

According to one embodiment, these rules are matched in such a way that,in the two rules, each code word is assigned a weighting vectorcorresponding to a transmission path of the first radio cell and aweighting vector corresponding to a transmission path of the secondradio cell.

The number of vectors originating from measurements on transmissionpaths of a particular radio cell in the set of weighting vectors may bedifferent for the individual radio cells; in particular, it may begreater than 1 for at least one radio cell and equal to 1 for at leastone other. It is therefore advisable, according to a second, if at leastone base station uses a rule in which each code word is assigned adifferent weighting vector, and at least one other base station usesanother rule whereby each code word is assigned the same weightingvector. Thus the subscriber station can use diversity at one basestation by transmitting different code words, while the other alwayssimultaneously employs the same weighting vector generally correspondingto the best transmission path available to that station.

Another advantageous possibility is that at least one of the basestations uses a first rule in which a number of code words are assignedto an identical weighting vector, and that at least one other basestation uses a second rule in which this number of code words isassigned different weighting vectors in each case. In particular, thecode words may contain first and second bits, the first bits in eachcase specifying the weighting vector used by one base station and thesecond bits specifying the weighting vector used by the second basestation. Thus, the subscriber station can use diversity at each of thebase stations even though the number of weighting vectors available forselection at each base station may be less than in the case of the twoabovementioned alternatives.

The weighting vectors at the mobile station are preferably specified bycalculating a covariance matrix for each base station and selecting atleast one weighting vector from the eigenvectors of each covariancematrix, the vectors selected being generally those whose eigenvectorshave the highest absolute value.

If the beam formation control method described above is used in thecontext of a soft handover between the first and second radio cell, inthe simplest case the set of weighting vectors may only include twovectors, and of the two code words required to designate these weightingvectors, it is the code word corresponding to the weighting vectorassigned to the first radio cell at the base station of the first radiocell and corresponding to the weighting vector assigned the second radiocell at the base station of the second radio cell that is transmitted tothe base stations. The selection of the weighting vectors used by thebase stations then preferably remains unchanged for the duration of thehandover, as transmission of the other code word in each case wouldresult in the base station of the first radio cell using a weightingvector specified on the basis of the downlink signal of the second radiocell, and vice versa. As this is unlikely to result in improvedreception, it is advisable not to change over the weighting vectors atthe base stations for the duration of the handover.

It can be provided that, when handover is complete, the base station ofthe second radio cell retains the second rule for assigning theweighting vectors to the code words transmitted by the mobile station.In such a case, in the event of a new handover to a third radio cell,its base station, in turn, applies the first rule.

An alternative possibility is for the base station of the radio cell togo over to using the first rule once handover is complete. Thisalternative is easier to implement, as a base station which begins tocommunicate with a mobile station as part of a handover can then use thesecond rule in every case, without it being necessary to clarify whichrule the other base station is using.

A base station suitable for implementing the method of the presentinvention is characterized in that it is set up to selectively use oneof two or more different rules for selecting the weighting vectors onthe basis of the code word transmitted. If only two stations areinvolved in a soft handover, two rules suffice; if a larger number ofbase stations are involved, a correspondingly larger number of rules arerequired.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the Figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 schematically illustrates beam formation in a conventional mobilecommunication system whose base stations each have two antennas andradiate a downlink signal over a relatively wide solid angle.

FIG. 2 shows a mobile radio communication system similar to thatillustrated to FIG. 1, with base stations having relatively tightlyfocused radiation characteristics;

FIG. 3 shows a mobile radio communication system similar to thatillustrated to FIG. 1 in which the method according to the presentinvention is used.

FIG. 4 schematically illustrates a first embodiment of the transmissionof weighting vectors measured at a mobile station from the mobilestation to two base stations and their processing in the base stations.

FIG. 5 schematically illustrates the transmission and processing ofweighting vectors according to a second embodiment.

FIG. 6 schematically illustrates the transmission and processing ofweighting vectors according to a third embodiment.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 shows a mobile station MS at the boundary between two radio cellsC1, C2 of a mobile radio communication system. The radio cells C1, C2are supplied by base stations BS1, BS2 which are interconnected via adata transmission network, also known as a core network (not shown inFIG. 3), via which they can exchange data with other base stations ofthe radio communication system or with a second telecommunicationnetwork connected to the data transmission network, or even jointlyreceive identical data intended for the same mobile station MS.

The two base stations BS1, BS2 use different scrambling codes fordownlink transmission to the mobile stations in their relevant cells C1,C2 which enable the mobile station MS, in the case of a downlink signalreceived by it, to detect whether it originates from base station BS1 orBS2.

This enables the mobile station MS to assess the transmission channelsbetween it and the two base stations BS1, BS2 individually and to createa covariance matrix R1, R2 for each base station individually. Thecovariance matrices are averaged over a sufficiently long period so thatthey are essentially independent of variations in reception conditionscaused by short-term interference phenomena. Eigenvector analysis of theaveraged covariance matrices performed by the mobile station MS providesa set of eigenvectors W_(i),_(BSj), i=1,2, . . . , M, where M is thenumber of antennas of the base stations BS1, BS2, j=1,2. For eachcovariance matrix the eigenvector with the largest eigenvalue isselected, and the two eigenvectors thereby obtained are transmitted tothe base stations BS1, BS2 as weighting vectors.

The transmission of the weighting vectors takes place in intervalsranging from a few seconds to minutes, as the base vectors change at arelatively slow rate depending of the movement speed of the mobilestation. Between two base vector transmissions, the mobile stationtransmits only code words, here 1 bit long, which indicate to the basestations which of the two weighting vectors they are to use fortransmitting to the mobile station.

After transmission of the weighting vectors, each base station BS1, BS2therefore has the same set of two weighting vectors, one determined onthe basis of its own downlink signal and the other originating from thedownlink signal of the other base station. The radiation characteristicsP1,BS1, P1,BS2, P2,BS1 and P2,BS2 of the two base stations BS1, BS2correspond to the two base vectors. The characteristics P1,BS1 andP2,BS2 which correspond in each case to the use of an eigenvector by thebase station on the basis of whose signal it was measured, are optimumfor the mobile station MS, the other two characteristics are obviouslyunsuitable for communication with the mobile station MS.

As long as a mobile station is located in the core area of a cell (e.g.,of the cell C1), and communicates exclusively with its base station BS1,both of the weighting vectors transmitted by the mobile station to thebase station BS1 originate from measurements on the downlink signal ofthe base station BS1. By transmitting a one-bit code word, the mobilestation MS can, in this case, specify to the base station BS1 which ofthe two weighting vectors it is to use in an upcoming timeslot of thedownlink signal. For this purpose there exists an assignment rulebetween code word and weighting vector used; e.g., such that of twoweighting vectors consecutively transmitted to the base station, thevalue “0” of the code word specifies the first and the value “1” thesecond.

As far as possible, an assignment rule of this kind must continue toapply to the base station BS1 even if, in the context of a handover, oneof the two weighting vectors periodically transmitted from the mobilestation to the two base stations involved is replaced by a weightingvector based on the downlink signal of the target base station BS2.

If the same assignment rule were to apply to the target base stationBS2, in the case of the example considered here this would result in thetwo base stations, at a given point in time, using either thecharacteristics P1,BS2 and P2,BS2 or P1,BS1 and P2,BS1, with the resultthat only the downlink signal of one base station would be receivable atthe subscriber station MS. To prevent this, the target base station BS2must apply another assignment rule which in the basic example consideredhere may only state that the base station BS2 shall use the secondtransmitted weighting vector if the feedback word has the value “0” andthe first transmitted weighting vector in the event of a feedback word“1”. As such, by transmitting an identical feedback word to both basestations, the latter can use different weighting vectors.

If it is assumed that the mobile station MS first transmits theweighting vector measured for the original base station BS1 and then theweighting vector measured for the target base station BS2, transmissionof the code word “0” then causes the base stations in each case to usethe weighting vectors corresponding to the radiation characteristicsP1,BS1, P2,BS2. As transmission of the code word “1” promises noimprovement in reception, the same feedback code word “0” is transmittedfor the entire duration of the soft handover.

To ensure that the target base station BS2 uses the correct assignmentrule, it suffices for it to be suitably signaled to the effect that, forthe connection to be established with the mobile station MS, it is thecase of a handover and not of a new call setup. In the case of a newcall setup, it must use the same first assignment rule as the basestation BS1. When handover is complete, the base station BS2 also goesover to using the first assignment rule in communication with the mobilestation MS. This takes place in dialog with the mobile station MS sothat the latter can adjust to this and correctly select the code wordssent to the base station BS2. A suitable time for changing theassignment rule, for example, is when, after handover, the mobilestation MS first transmits an updated set of weighting vectors to thebase station BS2. This procedure subsequently enables a new softhandover to another base station using exactly the same method asdescribed above.

Alternatively, it is also possible for the base station BS2 to use thesecond assignment rule throughout its communication with the mobilestation MS. In this case, however, it is necessary that, as part of thehandover, information concerning the assignment rule used by theoriginal base station BS1 be transmitted to the target base station BS2to enable it to correctly select the assignment rule used by it. Thus,if a second handover from the base station BS2 to a third base stationhas to take place, this third base station has to “know” that BS2 isusing the second rule in order, for its part, to be able to again selectthe first rule.

FIG. 4 illustrates a soft handover in a mobile communication system inwhich more than two weighting vectors are measured by the mobile stationMS and transmitted to base stations communicating with it. The weightingvectors are determined as described above. The weighting vectors aretransmitted to two base stations BS1, BS2 involved in a soft handover ina fixed sequence, odd-numbered positions 1, 3, . . . of the sequencebeing reserved for the transmission of weighting vectors W₁,_(BS1),W₂,_(BS1), . . . based on the downlink signal of the original basestation BS1 and weighting vectors W₁,_(BS2), W₂,_(BS2), . . . based onthe downlink signal of the target base station BS2 being transmitted ateven-numbered positions 2, 4, . . . . The original base station BS1stores the received weighting vectors in the order in which they arereceived and assigns them code words with the numerical values 0, 1, 2,. . . in this sequence. The target base station BS2, on the other hand,performs pairwise transposition of the weighting vectors, so that thecode words 0, 1, 2, 3 correspond to weighting vectors W₁,_(BS2),W₁,_(BS1), W₂,_(BS2), W₂,_(BS1), etc. With this system, an albeitlimited space diversity also can be used during soft handover; the codewords 0, 2, 4, . . . correspond to suitable combinations of weightingvectors in the sense that each base station uses a weighting vectorwhich has been defined on the basis of its own downlink signal.

FIG. 5 shows a variant of the transmission and processing of theweighting vectors. In the system considered here, four weighting vectorsW_(i),_(BSj) i=1, 2, j=1, 2 are measured which stand for the two besttransmission paths of each base station BSj to the subscriber stationMS. The weighting vectors are written to four storage locations of thebase stations in the sequence in which they are transmitted. To selectthe weighting vectors to be used by the base stations, the subscriberstation MS transmits four-bit code words.

Of these, the original base station BS1 analyzes the first two in eachcase; the last two are ignored, which in FIG. 5 is symbolicallyrepresented by letters xx at the corresponding positions of the codeword. The target base station BS2,conversely, only analyzes the last twobits of the code word and ignores the first two; i.e., there are intotal 16 code words of which four in each case specify the weightingvector at one of the two base stations BS1, BS2.

FIG. 6 shows a situation in which a mobile station simultaneouslyreceives a downlink signal from three base stations. It transmitsweighting vectors determined on the basis of these downlink signalstogether with an identifier BS1:, BS2: or BS3: which specifies, for eachvector, the base station on the basis of whose downlink signal it wasmeasured. The weighting vectors (four in this example) are received byall three base stations, the base stations detecting from the number ofidentifiers relating to them whether or not the subscriber stationrequires them to transmit with changing weighting vectors: the basestations BS2, BS3 which in each case receive only one weighting vectorrelating to them enter this weighting vector W1,BS2 or W1,BS3 at leastat the locations 0 and 1 of their store. The base station BS1, on theother hand, which has received more than one weighting vector relatingto it, detects from this that it is to employ diversity and enters thetwo vectors W1,BS1, W2,BS1 at the locations 0, 1 of its store. Using a1-bit code word 0 or 1, the subscriber station may now select from thetwo vectors W1,BS1, W2,BS1 at the base station BS1, whereas the otherbase stations always use the vector W1,BS2 or W1,BS3 determined as beingoptimum for them. The option of selecting between operation withchanging weighting vectors for one base station and operation with asingle weighting vector for another base station also is, of course,possible in a scenario where there are only two base stations, if one ofthe stations only receives one vector relating to it and all the othervectors relate to the other station, or if two identical weightingvectors are transmitted to a base station.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the present invention as set forth in the hereafter appended claims.

1. A method for controlling beam formation of a downlink signal radiatedfrom base stations of respective first and second radio cells to amobile station in a mobile radio communication system, the base stationrespectively having a plurality of antennas and being set up forweighted radiation of a downlink signal to the mobile station using aweighting vector, the method comprising the steps of: specifying a setof weighting vectors at the mobile station and the base stations of thefirst and second radio cells, each weighting vector being assigned atleast one code word; selecting an optimum weighting factor at the mobilestation and transmitting a code word assigned to the optimum weightingvector to the base stations; and performing weighted radiation of thedownlink signal using the weighting vector assigned to the code word ateach of the base stations; wherein the weighting vectors are differentlyassigned to the code words at the base stations of the first and secondradio cells so that the base stations weight the downlink signal usingdifferent weighting vectors on receipt of an identical code word, withat least some of the weighting vectors being defined to initiatesimultaneous transmission of downlink signals via the plurality ofantennas of a base station.
 2. A method for controlling beam formationof a downlink signal as claimed in claim 1, wherein the set of weightingvectors is defined to respectively contain at least one weighting vectorcorresponding to a transmission path of the first radio cell and atleast one weighting vector corresponding to a transmission path of thesecond radio cell, and wherein an identical code word is assigned to theweighting vector assigned to the first radio cell at the base station ofthe first radio cell and to the weighting vector assigned to the secondradio cell at the base station of the second radio cell.
 3. A method forcontrolling beam formation of a downlink signal as claimed in claim 2,wherein the downlink signal is additionally radiated by a base stationof at least a third radio cell, with the set of weighting vectorsincluding at least one further weighting vector corresponding to atransmission path of the at least third radio cell and with the at leastone further weighting vector being assigned the same code word at thebase station of the at least third radio cell.
 4. A method forcontrolling beam formation of a downlink signal as claimed in claim 3,wherein to define the set of weighting vectors, weighting vectors aremeasured at the mobile station and transmitted in sequence to the basestations, with a first group of positions in the sequence being reservedfor the transmission of weighting vectors corresponding to atransmission path of the first radio cell, and another group ofpositions being reserved for the transmission of weighting vectorscorresponding to a transmission path of another radio cell.
 5. A methodfor controlling beam formation of a downlink signal as claimed in claim4, wherein the mobile station transmits information concerning a numberof groups to the base stations.
 6. A method for controlling beamformation of a downlink signal as claimed in claim 4, wherein thepositions of the various groups follow one another cyclically in thesequence.
 7. A method for controlling beam formation of a downlinksignal as claimed in claim 2, wherein to define the set of weightingvectors, weighting vectors are measured at the mobile station andrespectively transmitted to the base stations in conjunction with anindicator designating the radio cell of the transmission path to whichthe transmitted weighting vector corresponds.
 8. A method forcontrolling beam formation of a downlink signal as claimed in claim 2,wherein the base stations employ two different rules for assigning aweighting vector to a code word transmitted by the mobile station.
 9. Amethod for controlling beam formation of a downlink signal as claimed inclaim 8, wherein in the two rules, each code word is assigned aweighting vector corresponding to a transmission path of one radio celland a weighting vector corresponding to a transmission path of the otherradio cell.
 10. A method for controlling beam formation of a downlinksignal as claimed in claim 8, wherein at least one of the base stationsemploys a first rule in which each code word is assigned a differentweighting factor, and wherein at least one other base station employs asecond rule whereby each code word is assigned the same weightingvector.
 11. A method for controlling beam formation of a downlink signalas claimed in claim 10, wherein the set of weighting vectors includestwo vectors, the code word corresponding to the weighting vectorassigned to the first radio cell at the base station of the first radiocell and corresponding to the weighting vector assigned the second radiocell at the base station of the second radio cell that is transmitted tothe base station, and thereafter for duration of the hand over, theselection of weighting vectors used by the base stations remainsunchanged.
 12. A method for controlling beam formation of a downlinksignal as claimed in claim 10, wherein when handover is complete, thebase station of the second radio cell continues to use the second ruleand, upon a further handover to a third radio cell, the base station ofthe second radio cell again uses the first rule.
 13. A method forcontrolling beam formation of a downlink signal as claimed in claim 10,wherein when handover is complete, the base station of the second radiocell switches to use the first rule.
 14. A method for controlling beamformation of a downlink signal as claimed in claim 8, wherein at leastone of the base stations employs a first rule in which a plurality ofcode words are assigned to an identical weighting vector, and wherein atleast one other base station employs a second rule in which theplurality of code words are assigned different weighting vectors in eachcase.
 15. A method for controlling beam formation of a downlink signalas claimed in claim 14, wherein the code words include first and secondbits, with the first bits specifying the weighting vector used by onebase station and the second bits specifying the weighting vector used byanother base station.
 16. A method for controlling beam formation of adownlink signal as claimed in claim 14, wherein when handover iscomplete, the base station of the second radio cell continues to use thesecond rule and, upon a further handover to a third radio cell, the basestation of the second radio cell again uses the first rule.
 17. A methodfor controlling beam formation of a downlink signal as claimed in claim14, wherein when handover is complete, the base station of the secondradio cell switches to use the first rule.
 18. A method for controllingbeam formation of a downlink signal as claimed in claim 1, wherein atthe mobile station, a covariance matrix is computed for each basestation and at least one weighting vector from eigenvectors of eachcovariance matrix is specified.
 19. A method for controlling beamformation of a downlink signal as claimed in claim 1, wherein the methodis used in connection with a soft handover between the first and secondradio cells.
 20. A base station in a mobile radio communication systemfor use in a method for controlling beam formation of a downlink signalradiated from base stations of respective first and second radio cellsto a mobile station in the mobile radio communication system,comprising: a plurality of antennas; parts for weighting radiation of adownlink signal to the mobile station using a weighting vector selectedon a basis of a code word transmitted by the mobile station; and partsfor selectively using one of at least two different rules for selectingthe weighting vectors based on the transmitted code word, the rule to beused being signaled to the base station via the mobile station.
 21. Abase station for a mobile radio communication system as claimed in claim20, wherein the base station may be used together with at least oneother base station, with the base station and the at least one otherbase station employing different rules.